Polar codes are the most recent breakthrough in coding theory. Polar codes are the first and, currently, the only family of codes with explicit construction to achieve the capacity of a certain family of channels (i.e., binary input symmetric discrete memoryless channels) as the block length goes to infinity. They have encoding and decoding algorithms with very low complexity. However, at moderate block lengths, their performance does not compete with previously known codes, which prevents them from being implemented in practice.